Electric power steering systems are known as in-vehicle systems installed in vehicles. Such an electric power steering system is operative to assist a driver's turning effort of a steering wheel using torque generated by a motor. For example, Japanese Patent Application Publication 2013-159289, referred to as a published patent document, discloses an example of these electric power steering systems. The electric power steering system disclosed in the published patent document includes an inverter comprised of three pairs of high- and low-side field-effect transistors (FETs) for controlling driving of a motor, such as a three-phase alternating-current (AC) motor.
Such an electric power steering system installed in a vehicle shares a battery serving as its power source with other external components. For example, if the electric power steering system and a starter share a battery serving as their power source, the voltage across the battery may rapidly decrease due to cranking of an internal combustion engine by the starter.
From this viewpoint, the electric power steering system disclosed in the published patent document includes three pairs of Zener diodes. The first pair of Zener diodes is connected between the gate and source of the first high-side FET, and the second pair of Zener diodes is connected between the gate and source of the second high-side FET. Additionally, the third pair of Zener diodes is connected between the gate and source of the third high-side FET.
Specifically, a decrease in the voltage across the battery may result in a difference in potential between the voltage across the battery and the source voltage of each high-side FET. At that time, each pair of Zener diodes is configured to protect the gate of a corresponding one of the high-side FETs even if the source-gate voltage of the corresponding high-side FET increases.